So, 5e has a much shallower power curve than 3e does. In 3e, you are supposed to be 2x as powerful every level (based off encounter building rules). In 5e, power is far closer to linear (a party of 4 level 20 PCs finds 29 CR 1 monsters to be a medium difficulty fight based off encounter building; a party of 4 level 1 PCs finds 1 CR 1 monster to be medium difficulty).
And fictionally, level 1 PCs are roughly as powerful.
So a 3e level 11 wizard is 32 times more powerful than a level 1 wizard, so is above level 20 in 5e. Throw in the fact that wizards scale too fast in 3e, and the things a level 10 wizard in 3e can do are easily those of post-level 20 "NPC" equivalents.
It isn't surprising that a story about a being whose power rivals 5e avatars of gods isn't well modeled by a level 10 5e wizard.